A method of labeling sensitive biological molecules such as proteins with tritium has been developed. The method consists of bombarding the sample, which has been lypholized or evaporated on a stainless steel plate, in a vacuum with vibrationally excited tritium species having high kinetic energies. Reaction of these excited species with the solid sample to be tritiated occurs in a random fashion with both exchangeable and non-exchangeable protons. After tritiation the exchangeable tritium atoms are removed from the sample by treatment with water. The reaction can be applied to any organic molecule which has accessible C-H bonds. The tritiation process is rapid and does not lead to decomposition of appreciable amounts of sample as is observed with other methods. The tritiation method has been applied to proteins such as thermolysin, ribionuclease, soybean trypsin inhibitor, elastase, and alpha1-antitrypsin. Specific activities in the range of 50-900 curie/mole have been obtained. Small molecules such as desmosine, nicotine sulfate, phenyl propylamine and leupeptin have also been tritiated to specific activities in the range of 0.05-60 curies/mole. Future research will be directed toward investigation of the parameters and the mechanism of the reaction in order to increase the degree of tritiation and discover any limitations to the method.